Nano-filter and method of forming same, and method of filtration

ABSTRACT

The disclosure relates generally to nano-filters and methods of forming same, and methods of filtration. The nano-filter includes a substrate and at least one nanowire structure located between an inlet and an outlet. The nanowire structure may include a plurality of vertically stacked horizontal nanowires.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of co-pending U.S. patent applicationSer. No. 12/966,370.

TECHNICAL FIELD

The disclosure relates generally to a nano-filter and a method offorming same, and a method of filtration, and more particularly, to anano-filter having a plurality of vertically stacked horizontalnanowires.

BACKGROUND

In separation technology, separation and identification of smallmolecules from larger ones is often performed. Separation may beemployed, for example, in biology and chemical analysis systems. Oftenfilters of various types are used with the aforementioned systems to aidin separation or analysis of samples.

SUMMARY

An aspect of the present invention relates to a nano-filter comprising:a substrate; and at least one nanowire structure on the substratelocated between an inlet and an outlet, wherein the nanowire structurecomprises a plurality of vertically stacked horizontal nanowires.

A second aspect of the present invention relates to a method of forminga nano-filter, the method comprising: forming at least one nanowirestructure having a plurality of vertically stacked horizontal nanowireson a substrate, the forming of the at least one nanowire structurecomprising: forming a plurality of layers of a first material and asecond material on the substrate; masking and etching through theplurality of layers of the first material and the second material; andremoving the plurality of layers of the second material selective to thefirst material so as to form the at least one nanowire structure havingthe plurality of vertically stacked horizontal nanowires; and depositinga capping layer on the at least one nanostructure so as to form thenano-filter.

A third aspect of the present invention relates to a method offiltration, the method comprising: passing a material through anano-filter comprising: a substrate; and at least one nanowire structureon the substrate located between an inlet and an outlet, wherein thenanowire structure comprises a plurality of vertically stackedhorizontal nanowires; and collecting a filtrate.

The illustrative aspects of the present invention are designed to solvethe problems herein described and/or other problems not discussed.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of this invention will be more readilyunderstood from the following detailed description of the variousaspects of the invention taken in conjunction with the accompanyingdrawings that depict various embodiments of the invention, in which:

FIG. 1 depicts a top-down view of an embodiment of a nano-filter, inaccordance with the present invention;

FIG. 2 depicts a cross-sectional view of an embodiment of a nanowirestructure, in accordance with the present invention;

FIG. 3 depicts a top-down view of another embodiment of a nano-filter,in accordance with the present invention;

FIG. 4 depicts a top-down view of another embodiment of a nano-filter,in accordance with the present invention;

FIG. 5 depicts a flow diagram of an embodiment of a method of forming anano-filter, in accordance with the present invention; and

FIGS. 6-8 depict cross-sectional views of an embodiment of steps of amethod of forming a nanowire structure, in accordance with the presentinvention.

It is noted that the drawings of the invention are not to scale. Thedrawings are intended to depict only typical aspects of the invention,and therefore should not be considered as limiting the scope of theinvention. In the drawings, like numbering represents like elementsbetween the drawings.

DETAILED DESCRIPTION

It has been discovered in biology analysis systems that detection of lowconcentration biomarkers may be limited due to the presence of largerbiomolecules in a fluid sample. Pre-fractionation and separation of thefluid sample may eliminate the larger biomolecules to enhance thedetection ability but none of the conventional separation techniques areappropriate for the task. Micro/nano fluidic molecular sievingstructures fabricated with semiconductor technology have been used toseparate biomolecules as well, though the systems have only beensuccessfully used for large biomolecule separation, such as viral DNA.

It also has been discovered that in chemical analysis systems, such asmass spectrometry systems, most systems are too big to accommodatemicroelectromechanical systems (MEMS). Typically, an interface is neededto connect the two. Such interfaces lead to the mass spectrometry systemdeveloping disadvantages such as dead sample volume which can obviatethe advantages gained in MEMS miniaturization and MEMS clogging.

Referring to FIG. 1, a top-down view of an embodiment of a nano-filter10 is shown. Nano-filter 10 comprises a nanowire structure 20, an inlet25, and an outlet 30. Inlet 25 may be operatively attached to nanowirestructure 20 such that inlet 25 may allow a material to flow intonanowire structure 20 wherein the material may be eventually filtered.Outlet 30 may also be operatively attached to nanowire structure 20 suchthat outlet 30 allows the filtered material to flow out of nanowirestructure 20. In an embodiment, inlet 25 and outlet 30 may be respectiveopenings of nanowire structure 20 and not separate elements. Methods ofoperatively attaching a separate inlet and outlet to a nano-filter asdescribed herein are well known in the art. As shown in across-sectional view of FIG. 2, nano-filter 10 may additionally comprisea substrate 100 and a capping layer 60. Substrate 100 may be a base onwhich nano-filter 10 is supported. Capping layer 60 may be a top layerthat covers nanowire structure 20 as well as inlet 25 and outlet 30. Asshown in FIG. 1, nanowire structure 20 may have a depth d. Arrows 26indicate the direction of flow through nano-filter 10 and in particular,nanowire structure 20.

Referring to again to FIG. 2, nanowire structure 20 may comprise aplurality of vertically stacked horizontal nanowires 50, capping layer60, silicon layers 70, silicon-germanium layers 80, openings 90, andsubstrate 100. The material for filtration enters openings 90 in thedirection indicated by directional arrows 26 (see FIG. 1). The presentdiscussion will focus on an embodiment wherein the nanowire structure isas shown in FIGS. 1 and 2. The discussion is also equally applicable toany embodiment of nanowire structures (21, 22, 40, 42, 44, and 46)described herein (see FIGS. 3 and 4).

Plurality of vertically stacked horizontal nanowires 50 may besubstantially parallel to substrate 100 and may be substantiallyparallel to each other. Nanowires 50 may also be vertically stacked ontop of each other with openings 90 in between that extend the entiredepth d of nanowire structure 20 (see FIG. 1). In an embodiment,nanowires 50 may comprise silicon. In another embodiment, nanowires 50may comprise the same material that layers 70 comprise. Nanowires 50 maybe uniformly spaced, i.e., openings 90 may have similar widths.Alternatively, may be non-uniformly spaced openings 90, i.e., openings90 may have different widths. Openings 90 may be located betweennanowires 50, and any of silicon layer 70, and substrate 100. In anembodiment, openings 90 may also be located between nanowires 50 andcapping layer 60. Openings 90 may comprise open space.

Capping layer 60 may be the top layer of nanostructure 20 and may coverplurality of vertically stacked horizontal nanowires 50. In anembodiment, capping layer 60 may also be the top layer of nano-filter 10as well as the top layer of nanostructure 20 simultaneously. Cappinglayer 60 may include but is not limited to a material such as silicon orsilicon nitride. Capping layer 60 and materials comprising them areknown in the art.

Substrate 100 may be a base on which nano-filter 10 is supported.Substrate 100 may be integrally attached to nanowire structure 20, i.e.,substrate 100 may be a non-discernible element of nanowire structure 20.Substrate 100 may also be non-integrally attached to nanostructure 20,i.e., substrate 100 may be a separate, discernible element of nanowirestructure 20. In an embodiment, substrate 100 may be a semiconductorsubstrate such as but not limited to silicon, germanium, silicongermanium, silicon carbide, and those consisting essentially of one ormore Group III-V compound semiconductors having a composition defined bythe formula Al_(X1)Ga_(X2)In_(X3)As_(Y1)P_(Y2)N_(Y3)Sb_(Y4), where X1,X2, X3, Y1, Y2, Y3, and Y4 represent relative proportions, each greaterthan or equal to zero and X1+X2+X3+Y1+Y2+Y3+Y4=1 (1 being the totalrelative mole quantity).

Semiconductor substrate 100 may also comprise Group II-VI compoundsemiconductors having a composition Zn_(A1)Cd_(A2)Se_(B1)Te_(B2), whereA1, A2, B1, and B2 are relative proportions each greater than or equalto zero and A1+A2+B1+B2=1 (1 being a total mole quantity). The processesto provide semiconductor substrate 100 described herein are well knownin the art. In an embodiment, semiconductor substrate 100 may comprise ap-type doped substrate. Examples of p-type dopants include but are notlimited to boron (B), indium (In), and gallium (Ga). In anotherembodiment, semiconductor substrate 100 may comprise an n-type dopedsubstrate. Examples of n-type dopants include but are not limited tophosphorous (P), arsenic (As), and antimony (Sb).

As shown in FIG. 2, the ratio of width w to height h of nanowirestructure 20 may be in a range from approximately 3:1 to approximately100:1. The ratio of width w to depth d (see FIG. 1) of nanowirestructure 20 may be in a range from approximately 1:1 to approximately100:1.

Referring to FIG. 3, a top-down view of another embodiment of anano-filter is shown. Nano-filter 10 may comprise multiple nanowirestructures 20, 21, and 22 described herein. The material for filtration,as indicated by directional arrows 26, may enter nano-filter 10 viainlet 25. The material may then continue to and through nanostructures20, 21, and 22 for filtration and eventually exit nano-filter 10 viaoutlet 30. Nano-filter 10 may additionally comprise substrate 100 andcapping layer 60 as shown in FIG. 2. Capping layer 60 may be a top layerthat covers nano-filter 10 as well as nanowire structures 20, 21, and22.

Referring to FIG. 4, a top-down view of another embodiment of anano-filter is shown. Nano-filter 10 may comprise multiple nanowirestructures 40, 42, 44, and 46 described herein. Nanowire structures 40,42, 44, and 46 may have different depths d, and may be of various shapesand sizes. Nanowire structures 40, 42, 44, and 46 may independentlycomprise openings 90 (see FIG. 2) having different widths to filterdifferent size materials.

Referring to FIGS. 5-8, an embodiment of a method of forming anano-filter 10 is presented. Step S1, forming at least one nanowirestructure 20 on a substrate 100 comprises steps S1A to S1C. Step S1A, asshown in FIG. 6, may include forming a plurality of layers of a firstmaterial 70 and a second material 80 on substrate 100. In an embodiment,first material 70 may comprise silicon and second material 80 maycomprise silicon-germanium. Embodiments of substrate 100 have beenpreviously described herein. In an embodiment, substrate 100 may besilicon.

Plurality of layers of first material 70 and second material 80 may bedeposited on semiconductor substrate 100 using known techniques in theart. In an embodiment, plurality of layers of first material 70 andsecond material 80 may be alternating. The thickness of second materiallayers 80 may be in a range from approximately 2 nm to approximately 10microns and all subranges therebetween. Plurality of layers of firstmaterial 70 and second material 80 deposited may number fromapproximately 2 to approximately 100.

Examples of deposition techniques may include but are not limited to:chemical vapor deposition (CVD), low-pressure CVD (LPCVD),plasma-enhanced CVD (PECVD), semi-atmosphere CVD (SACVD) and highdensity plasma CVD (HDPCVD), rapid thermal CVD (RTCVD), ultra-highvacuum CVD (UHVCVD), limited reaction processing CVD (LRPCVD), metalorganic CVD (MOCVD), sputtering deposition, ion beam deposition,electron beam deposition, laser assisted deposition, thermal oxidation,thermal nitridation, spin-on methods, physical vapor deposition (PVD),atomic layer deposition (ALD), chemical oxidation, molecular beamepitaxy (MBE), plating, and evaporation. Any later developed techniquesappropriate for the deposition of plurality of layers of first material70 and second material 80 on substrate 100 may also be used.

Step S1B, as shown in FIG. 7, may include masking and etching throughplurality of layers of first material 70 and second material 80 to formopenings 90. Masking and etching may be performed using any knowntechniques in the art or later developed techniques appropriate formasking and etching layers of first material 70 and second material 80,and in particular, when first material 70 may comprise silicon andsecond material 80 may comprise silicon-germanium. For example, ananisotropic reactive ion etch (RIE), e.g., fluorine-based etch, may beused to etch vertically through selected regions of the first material70 and material 80 layers.

As shown in FIG. 8, step S1C includes selectively removing a pluralityof second material layers 80 to form a plurality of vertically stackedhorizontal nanowires 50. Techniques for selectively removing secondmaterial layers 80, and in particular, silicon-germanium layers, areknown in the art. In one embodiment, the removing may be performed by anisotropic, selective etch of the exposed second material 80 layers ofstep S1B. The selective etch may comprise oxidation at approximately700° to approximately 900° C., which is known to oxidize, for example,silicon-germanium layers at a much more rapid rate than, for example,silicon layers. A dilute hydrogen fluoride (DHF) wet etch may then beperformed to remove the oxidized regions resulting in the formation ofhorizontal nanowires 50. Step S1C may further comprise etching pluralityof vertically stacked horizontal nanowires 50 again and as previouslydescribed so as to reduce their thickness and increase the width ofopenings 90. Plurality of vertically stacked horizontal nanowires 50 maynumber in a range from approximately 2 to approximately 100. Nanowires50 may have a thickness in a range from approximately 2 nm to 500 nm andsubranges therein. Nanowires 50 may also have a thickness of 2 nm to 50nm and subranges therein. Openings 90 may have a width in a range fromapproximately 5 nm to 15 microns. In another embodiment, openings 90 mayhave a width of approximately 10 nm to approximately 100 nm.

The width of openings 90 may be accurately controlled by masking andfurther etching of nanowires 50; masking, oxidizing, and stripping ofnanowires 50; and/or varying etch rates, all of which are known in theart. In an embodiment, the etch rate may be varied by changing theimplant angle when first material layers 70 and/or second materiallayers 80 are formed on substrate 100. Step S1C may further comprisemodifying plurality of vertically stacked horizontal nanowires 50 byoxidation, silicidation, metallization, and the like. Metals for use inthe metallization treatment may include but are not limited to aluminum,nickel, copper, titanium, tantalum, platinum, and tungsten.

Returning to FIG. 5, step S2, depositing capping layer 60 on the atleast one nanostructure 20 so as to form nano-filter 10 may be performedusing any known techniques in the art or later developed techniquesappropriate for depositing a capping layer. After formation, nanowirestructure 20 may be operatively integrated with complementary metaloxide semiconductor (CMOS) logic on a chip. Nanowire structure 20 may beintegrated with CMOS logic such that the CMOS log is planar with the topsurface of nano-filter 10. Methods for integrating CMOS logic withnanostructures are known in the art.

An embodiment of a method of filtration is presented. Referring to FIGS.1 and 2, a nano-filter 10 may be provided for filtration. Variousembodiments of nano-filter 10 have been previously described herein. Thepresent discussion will focus on nano-filter 10 having nanowirestructure 20 as is shown in FIGS. 1 and 2 but is applicable tonano-filter 10 having any of nanowire structures 21, 22, 42, 44, or 46(see FIGS. 3 and 4). A flow of material for filtration may beestablished with nano-filter 10 via an inlet 25. Inlet 25 may be coupledto various devices and systems that provide a flow of material forfiltration, for example, a microelectromechanical system (MEMS).Examples of MEMS include but are not limited to a molecular sorter and abiological sorter. The material for filtration may include but is notlimited to biomolecules, blood, DNA, proteins, drinking water, and thelike. Techniques for coupling inlet 25 to MEMS are known in the art.

Material, for example biomolecules, may flow into nano-filter 10 viainlet 25. The biomolecules may continue to flow and enter nanowirestructure 20 via openings 90, and pass between plurality of verticallystacked horizontal nano-wires 50. Biomolecules larger than openings 90may not pass through nanowire structure 20 being that the biomoleculesare larger than openings 90. Biomolecules smaller than openings 90 maypass through nanowire structure 20 resulting in the material beingfiltered and a filtrate exiting nano-filter 10 via outlet 30. Thefiltrate may be collected or transferred to another system via outlet30. Outlet 30 may be coupled to a non-MEMS. Examples of non-MEMS includebut are not limited to a mass spectrometer, a gel electrophoresisapparatus, a capillary electrophoresis apparatus, a genetic sequencer, anuclear magnetic resonance instrument, and etc. Methods of coupling anoutlet 30 to a non-MEMS described herein is well known in the art.

Another embodiment of a method of filtration is presented. Referring toFIGS. 2 and 3, a nano-filter 10 having multiple nanowire structures 20,21, and 22 may be provided for filtration. A flow of material, as shownby directional arrows 26, for filtration may be established withnano-filter 10 via an inlet 25. As described herein, material, forexample biomolecules, may flow into nano-filter 10 via inlet 25, and maycontinue to flow to be filtered by nanowire structure 22 based on thesize difference of openings 90 and the biomolecules to be filtered.

The nanowire structures 20-22 may have graduated openings 90.Biomolecules smaller than openings 90 may pass through nanowirestructure 20 and may continue to nanowire structure 21. Nanowirestructure 21 may have openings smaller than openings 90 of nanostructure20 and the biomolecules that passed through nanowire structure 20 may befurther filtered by nanowire structure 21. Biomolecules that passthrough nanowire structure 21 may also continue for further filtrationby nanowire structure 22 having openings that differ from those ofnanowire structures 20 and 21. Embodiments of nano-filter 10 of thepresent invention having multiple nanowire structures may providemultiple filtrations of a material passing therethrough by varying thewidths of the openings of the multiple nanowire structures.

Not being held to any particular theory, the filtration of the materialmay occur due to size exclusion, i.e., the molecules to be filtered arelarger then openings 90. Filtration may also occur via chemical sensing,i.e., nanostructures 20, 21, and 22 may independently have an appliedelectrical field in which plurality of nanowires 50 are electricallyactive and can be switched. Chemical sensing is known in the art.

The foregoing description of various aspects of the disclosure has beenpresented for purposes of illustration and description. It is notintended to be exhaustive or to limit the disclosure to the precise formdisclosed, and obviously, many modifications and variations arepossible. Such modifications and variations that may be apparent to aperson skilled in the art are intended to be included within the scopeof the disclosure as defined by the accompanying claims.

What is claimed is:
 1. A nano-filter comprising: a substrate; and atleast one nanowire structure on the substrate located between an inletand an outlet, wherein the nanowire structure comprises a plurality ofvertically stacked horizontal nanowires.
 2. The nano-filter according toclaim 1, wherein the plurality of vertically stacked horizontalnanowires comprise silicon.
 3. The nano-filter according to claim 1,wherein the plurality of vertically stacked horizontal nanowires numberin a range from approximately 2 to approximately
 100. 4. The nano-filteraccording to claim 1, wherein the plurality of vertically stackedhorizontal nanowires have a thickness in a range from approximately 2 nmto approximately 500 nm.
 5. The nano-filter according to claim 4,wherein each nanowire of the plurality of vertically stacked horizontalnanowires independently has a thickness in a range from approximately 2nm to approximately 50 nm.
 6. A nano-filter according to claim 1,wherein the plurality of vertically stacked horizontal nanowires ismodified by a treatment selected from one of metallization, oxidation,and silicidation.
 7. The nano-filter according to claim 6, wherein theplurality of vertically stacked horizontal nanowires is metalized withmetals selected from one of aluminum, nickel, copper, titanium,tantalum, platinum, and tungsten.
 8. The nano-filter according to claim1, wherein the plurality of vertically stacked horizontal nanowirescomprises an applied electric field.
 9. The nano-filter according toclaim 1, wherein the nanowire structure includes integration with acomplementary metal oxide semiconductor logic.
 10. The nano-filteraccording to claim 1, wherein the nanowire structure comprises anopening width to an opening height ratio of approximately 3:1 toapproximately 100:1.
 11. The nano-filter according to claim 1, whereinthe nanowire structure comprises an opening width to an opening depthratio of approximately 1:1 to approximately 100:1.
 12. The nano-filteraccording to claim 1, wherein the plurality of vertically stackedhorizontal nanowires are non-uniformly spaced.
 13. A method offiltration, the method comprising: passing a material through anano-filter comprising: a substrate; and at least one nanowire structureon the substrate located between an inlet and an outlet, wherein thenanowire structure comprises a plurality of vertically stackedhorizontal nanowires; and collecting a filtrate.
 14. The method forfiltration according to claim 13, wherein the plurality of verticallystacked horizontal nanowires have a thickness in a range fromapproximately 2 nm to approximately 500 nm.
 15. The method according toclaim 13, wherein the material is selected from one of biomolecules,blood, and drinking water.